Apparatus for maintaining the temperature and operating a calibrated lamp in a constant resistance mode

ABSTRACT

The resistance of an incandescent lamp filament, subject to ambient variations, is determined by sensing the current and voltage of the lamp filament. Since the resistance is a known function of the temperature of the filament, changes in the temperature of the filament are sensed to provide an output correction signal to a power supply which adjusts the input parameters of the lamp to maintain filament temperature constant.

United States Patent Percival T. Gates, Jr. Weston, Mass. 790,984

Jan. 14, 1969 Aug. 3, I971 EGltG, Inc. BedIord, Mass.

Inventor Appl: No. Filed Patented Assignee APPARATUS FOR MAINTAINING THETEMPERATURE AND OPERATING A CALIBRATED LAMP IN A CONSTANT [56]References Cited UNITED STATES PATENTS 2,054,496 9/1936 Craig 315/291 X3,202,870 8/1965 Carlon.... 315/116 3,263,125 7/1966 Genuit 315/291 X3,354,384 11/1967 Benjamin 315/311 X 3,474,343 10/1969 Smith 328/83,482,145 12/1969 Powell... 315/287 X 3,486,069 12/1969 Mahier 315/291 XPrimary Examiner-John W. Huckert Assistant ExnminerAndrew .1. JamesAttorneys-Ralph L. Cadwallader and Lawrence P. Benjamin ABSTRACT: Theresistance of an incandescent lamp filament, subject to ambientvariations, is determined by sensing the current and voltage of the lampfilament. Since the resistance is a known function of the temperature ofthe filament, changes in the temperature of the filament are sensed toprovide an output correction signal to a power supply which adjusts theinput parameters of the lamp to maintain filament 328/8 temperatureconstant,

r-1Z.1 @4 1 POWER SOURCE VOLTAGE SENSE 12.2

CURRENT SENSE 18/ I REGULATE PATENTED AUG 31911 597 65 sum 1 OF 2[--12.1 /12 3 14 POWER SOURCE VOLTAGE SENSE 122 CURRENT SENSE 18 LREGULATE Fig. 2.

REFERENCE 122.2

SENSITIVE -122.1 DR'VE DETECTOR /A\\132 |40\E AL AM l 134 i J PE RCIVALT GATES, Jr

PATENIEDIIUG 3I97l 3597552 SHEET 2 OF 2 H F v 4 :26]

I 2 20.5 k 241/? i TO D.C.

SOURCE APPARATUS F OR MAINTAINING THE TEMPERATURE AND OPERATING ACALIBRATED LAMP IN A CONSTANT RESISTANCE MODE BACKGROUND OF THEINVENTION This invention relates to regulating systems and moreparticularly to a system for maintaining the filament temperature ofacalibrated light source at a predetermined, constant value.

When standard incandescent lamps are calibrated by the National Bureauof Standards the Bureau provides the user of the lamps with a sheet ofinstructions and procedures detailing the conditions under which thelamp is to be operated, and typically may include a tabulation of thecalibrated light output for various filament currents. In some instancesvalues of spectral irradiance are tabulated as a function of wavelengthat a distance of 50 centimeters, as measured from the center of the lampto the receiver for a lamp current of 8.30 amperes.

The instructions cover the use of the particular filament lamp issued asstandards of spectral irradiance for a particular wavelength range. Thespectral irradiance from the lamp is based on the spectral radiance ofblackbody as defined by Plancks equation and has been determined throughcomparison ofa group of similar lamps with (I) the NBS standards ofspectral radiance, (2) the NBS standards ofluminous intensity, and (3)the NBS standards of total irradiance.

THe instructions provided are quite explicit and require that the lampbe mounted in a specific orientation in the supplied holder which isconstructed in such a manner as to reflect a negligible amount ofradiated flux in the direction of the receiver or spectrometer slit. Ablack shield must be placed at a distance of about 4 feet to the rear ofthe lamp to intercept stray radiant flux along the radiometric axis andadequate shielding must be provided to intercept stray flux from otherdirections. These procedures are intended to improve subsequent userreproducibility of calibrated output.

However, the lamp may be subject to environmental changes whichinfluence such reproducibility. For example. variations in temperatureof the atmosphere immediately surrounding the lamp envelope, orunpredictable minor movements of air will tend to produce a measurableand undesirable change in output of the lamp.

It is well known that when a lamp is being operated, a condition ofoverall thermal equilibrium is established. Both the filament and theenvelope achieve such conditions wherein power input is equal to poweroutput and the equilibrium is manifested in stable temperatures at .anypoint of both elements.

In the filament, the electrical power is converted to thermal powercausing filament temperature to increase until output power (radiated,convected, and conducted) is equal to input power. The envelope isheated primarily through radiation from the filament.

A condition of thermal equilibrium of the envelope is achieved when thepower received is balanced by the power transferred to the surroundings,such condition is again manifested by stable temperatures.

If now more convective power is transferred to the surroundings by theenvelope (for example, by moving air or reduced ambient temperature),envelope temperature will decrease, and by the laws of radiation powertransfer, more power will flow from filament to envelope. Underconditions of constant filament voltage or constant filament current,the resulting effect is reduced filament temperature. Since the opticaloutput power is a function of the absolute temperature of the filament,any reduction in filament temperature produces a corresponding reductionof light output representing a departure from the calibrated condition.

It can be proven that a blackbody operating at 3000" K. must have itstemperature maintained to an accuracy of i% K. to maintain the spectralirradiance to :0.1 percent at 400 nanometers. This corresponds to about10.01 percent of the resistance of a tungsten filament. It willtherefore be obvious that to maintain a constant and repeatable lightoutput, that I the filament temperature must be held constant.

The electrical resistance of a filament is a well established functionof its absolute temperature. Thus, a given filament temperaturecorresponds to a predictable and stable value of filament resistance.Therefore, reproducibility of light output is assured through operationat constant filament temperature which may be achieved by suchelectrical input adjustment as to maintain filament resistance constant.

It is well known that the resistance of a device is described by Ohmslaw, namely: R=E/I and that the resistance R is equal to the ratio ofvoltage-to-current used by the device. Therefore, one may determinefilament resistance (R) through sensing filament voltage (E) and current(I).

It is, therefore, one object of the present invention to provide meansfor controlling the power emitted from a resistive impedance element byutilizing the relationship between the resistance of the element and thetotal emissive power therefrom.

Another object of the present invention is to provide a control circuitfor controlling a standard, calibrated lamp utilizing the relationshipbetween the total emissive power or lamp brightness and the lampfilament resistance.

Still another objectof the present invention is to provide a controlsystem for detecting subtle changes in resistive impedance of acalibrated lamp due to changes in surrounding conditions.

Yet another object of the present invention is to provide a controlsystem for a standard calibrated lamp capable of detecting changes inambient conditions in the vicinity of the lamp by detecting changes inthe ratio of voltage-to-current (EU) or resistance of the filament.

The features of my invention which I believe to be novel are setforthwith particularlity in the appended claims. My invention itself,however, both as to its organization and method of operation, togetherwith further objects and advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawing.

DESCRIPTION OF THE DRAWINGS FIG. Us a plot of current and voltage acrossthe terminals ofan incandescent, light standard;

FIG. 2 is a block diagram incorporating the features of my inventionwherein a constant ratio of voltage to current is maintained in acalibrated lamp system;

FIG. 3 is a circuit diagram of one embodiment of a system incorporatingthe features of the invention for controlling the light from acalibrated light source by maintaining a constant ratio of voltage tocurrent; and

FIG. 4 is a circuit diagram of another embodiment of a systemincorporating the features of the invention for controlling the lightfrom a calibrated light source by maintaining a constant ratio ofvoltage to current.

SUMMARY OF THE INVENTION As will be hereinafter shown, my device sensesthe voltage applied to and the current passing through a hightemperature resistive impedance, such as calibrated lamp, and treats theratio of voltage to current sensed (EU) as the parameter most indicativeof the element temperature. This parameter, in the case ofa lampfilament, represents a measure of the brightness of the lamp. Bymaintaining this ratio of El] constant, in spite of variations inthe-lamp environment, I am able to achieve a constancy oflight outputfar exceeding that resulting from the recommended method involvingconstant filament current.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now. to FIG. 1, thereis shown a plot of current versus voltage wherein curve A may representthe current throughthe filamentover a range of values of filamentvoltage under a set of constant lamp environmental conditions. It may benoted that as current (or voltage) is increased, filament resistance isincreased as is evidenced by the curvature of the characteristic curve.At an opening point x, the filament is operating at a unique value ofresistance as shown by dashed line R. This resistance in turncorresponds to a unique and particular filament temperature. The presentinvention provides circuitry for operating a lamp filament at theintersection of its characteristic 5-! curve and line R of constantresistance.

Referring now to H6. 2, there is shown, in block diagram form, powersource 12.1 having leads 12.3 and 12.4 for supplying power to seriesconnected lamp 14. Voltage sensing means 16, connected in parallel withlamp 14, and current sensing means 18, connected in series with lamp 14,each is provided with an output which is applied as an input toregulator 20.1. Regulator 20.1 produces a signal voltage at output lead20.2 which is representative of the ratio of the voltage to the current(E/I) being used by lamp 14. This correction or error signal voltage isfed to power source 12.1 as a means for varying a parameter of lamp 14.A manual adjustment means 12.2 is provided in power source 12.1 toinitially set the operating conditions of the power source while theerror signal, appearing on lead 20.2 is the means for making a fineadjustment to maintain the El! ratio or resistance of lamp l4 constant.

Referring now to FIG. 3 there is shown another embodiment employing theprinciples of my invention which utilizes direct current (DC) means forcontrolling the parameters of lamp 14 so that it will maintain itsproper operating conditions. The components enclosed within dotted line40 represent a magnetic amplifier wherein first control winding 18,connected in series with lamp 14, represents the current sensing meansand second control winding 16.1, together with variable resistor 16.2 isconnected in parallel with lamp 14 and represents the voltage sensingmeans. As is well known in the art, magnetic amplifier 40 is providedwith an excitation means 20.5 having a primary winding connected toterminals 20.6 and 20.7 to which is connected a suitable source ofexcitation (not shown). Center-tapped secondary 20.8 has one end thereofconnected to the series connected load windings 20.2 and 20.3 and hasits other end connected to the series load windings 20.1 and 20.4. Eachof the load windings 20.1, 20.2, 20.3 and 20.4 is connected to one endof respective diodes 22.1, 22.2, 22.3 and 22.4. The other ends of diodes22.1 and 22.2 are connected to one end of load resistor 24.1 and thecommon junction of the diodes and resistor is connected to outputterminal 26.1. Similarly, the other ends of diodes 22.3 and 22.4 areconnected to one end ofload resistor 24.2 with the junction common tothese latter elements connected to output terminal 26.2. To complete thecircuit, the other ends of resistors 24.1 and 24.2 are connectedtogether and to the center tap of excitation winding 20.8.

In the operation of my device in this mode it will be seen that lamp 14is provided with the appropriate value ofcurrent and voltage, fromsource 12.1. Variable resistor 12.2 is provided in supply 12.1 forproviding the initial settings ofcurrent and voltage to maintain theproper operating conditions. If, after the initial parameters are set,the ambient conditions change and the lamp is now subjected to amovement of air sufficient to reduce the temperature ofthe lampenvelope, the operating conditions will change. Assume for example, thatas originally set up, the lamp is made to operate at a voltage andcurrent that would correspond to point x of curve A (FIG 1). 1f now theambient conditions change so that the lamp envelope temperature islowered, the temperature of the filament would also change as theequilibria are upset. lf now source 12.1 were a constant current source,it would be obvious that for the current to be maintained constant theoperating point would be shifted along line y to curve B so thatoperation would be at point 1 corresponding to a different filamentresistance and hence filament temperature. This would substantiallyalter the light output of lamp 14, as there is a lower voltage appearingacross the filament of the lamp and indicating a change in filamentresistance. However, the turns ratio of control windings 16.1 and 18together with resistance 16.2 were appropriately selected so that anychanges in sensed voltage to current ratio were manifested by theappearance of an error signal at the output terminals 26.1 and 26.2. Onemay then use the error signal to maintain a constant ratio of E/! orresistance. The windings may be arranged so that if a positive goingsignal were to appear at terminals 26.1 with respect to 26.2 it would beindicative ofa lamp load condition which indicates that the currentpassing through lamp 14 is too high. A negative voltage at the terminalswould indicate that the current is too low while the amplitude ofthesignal would be proportional to the error. This error signal is thenapplied to the base of transistor 12.3 to regulate the current andvoltage appearing at the output terminals of DC source 12.1 and therebymaintained the present E/I ratio so that lamp characteristic curve B(FIG. 1) will not be operated at point w.

It is known in the art that a magnetic amplifier of this type producesan output voltage proportional to the algebraic addition of theampere-turns of the central windings. In the system configuration ofFIG. 3, feedback control action tends to reduce the net ampere turns ofcontrol windings 16.1 and 18 toward zero. A null at output terminals26.1 and 26.2 corresponds to a constant ratio of the Eto l oflamp 14.

Referring now to FIG. 4, there is shown another embodiment employing theprinciple of my invention and which operates the lamp in an alternatingcurrent (AC) mode. In this embodiment transformer 140, indicated withindotted line is provided with winding 118 which represents the currentsensing means and winding 116.1 which, together with resistor 116.2,represents the voltage sensing means.

Operating potential is derived from autotransformer 136 which may be ofthe type known commercially as a Variac. A source of AC potential (notshown) is applied to terminals 138.1 and 138.2 so that the AC potentialappearing thereacross may be picked, at any suitable value, by moveablearm 134. This potential is then applied through current sensing winding118 which is in series with lamp load 114. Transformer 140 is furtherprovided with a center tapped sensing winding 120 for detecting changesin the sensed current and voltage used by lamp 114. When slight changesin the operating parameters occur, the change will be marked by theappearance of an AC error voltage in winding 120. The amplitude isproportional to the degree of unbalance. The voltage will be in phase or180 out of phase with the line according to the sense of the unbalance.The resulting error signal is then applied to a phase sensitive detector122.1, the output of which is a DC signal which may have, for example, apositive value when conditions indicate a high current through lamp 114and the negative value for low current through lamp 114. As in theprevious embodiment, the amplitude of the signal from detector 122.1will be proportional to the error. This output error signal is thenapplied as an input to a drive on amplifier means 128 which, in turn,provides a signal at its output, suitable to operate servo or motormeans 130. The shaft of servo means 130 is coupled to the moveable arm134 of autotransformer 136. as indicated by dotted lines 132, so thatrotation of the shaft of servo means 130 will, with the appearance ofanerror signal at the output ofdetector 122.1, appropriately vary theinput parameters to lamp 114 and thereby reduce the error signal to zeroand hence maintain the required constant E/l ratio.

While there has been described what is presently considered thepreferred embodiment of my invention, it would now be obvious to allthose skilled in the art that various other changes and modificationsmay be made therein without departing from the inventive conceptcontained herein and it is, therefore, aimed to cover all such changesand modifications as fall within the true spirit and scope ofmyinvention.

What 1 claim is:

1. Apparatus for maintaining the temperature of a lamp filamentconstant, comprising:

a source of potential connected to the filament for providing operatingparameters ofa given ratio thereto;

a plurality of sensing means connected in series and in parallel withthe filament;

each sensing means detecting an operating parameter of the filament, andeach deriving respective signal voltages therefrom representative of thesensed parameter;

means connecting the respective signal voltages to detecting means forsensing and for deriving an error signal in response to changes in theratio of sensed parameters; and

means connecting the error signal to the source of potential to varytheoperating parameters and maintain the ratio constant.

2. The apparatus of claim 1, wherein:

the potential connected to the lamp filament is direct current; and

the operating parameters are filament voltage and filament current.

3. The apparatus of claim 2, wherein;

the means sensing the voltage operating parameter and the means sensingthe current operating parameter are a pair of magnetic amplifier controlwindings; and

output windings coupled to the control windings for generating a signalvoltage representative of changes in the ratio of the sensed voltage andsensed current of the lamp filament.

4. The apparatus of claim 3, further comprising:

a constant current power supply providing the voltage and currentparameters; and

a series passing stage in series with the power supply output fordetecting the changes in the ratio of sensed voltage to sensed current.

5. The apparatus of claim 1, wherein:

the potential connected to the lamp filament is alternating current; andv the operating parameters are filament voltage and filament current.

6. The apparatus of claim 5, wherein:

the means sensing the voltage operating parameter and the means sensingthe current operating parameter are a pair of electromagneticallycoupled transformer windings; and

an output winding coupled to the pair of windings for generating asignal voltage representative of the ratio of the sensed voltage to thesensed current of the lamp filament.

7. The apparatus ofclaim 6, further comprising:

a variable output autotransformer, the input of which is connected tothe source of alternating current the output of which is connected toone pair of windings; and

drive means connected to the output of the autotransformer for varyingthe operating parameters to the transformer.

8. Tile apparatus of claim 7, further comprising:

a phase sensitive detector connected to the output winding:

a source of reference phase voltage connected to the phase detector;

the phase detector combining the signal voltage and the reference phasevoltage to derive an error signal responsive to the change in the ratioof sensed parameters; and

means connecting the output of the phase sensitive detector to the drivemeans to vary the operating parameters of the lamp filament.

1. Apparatus for maintaining the temperature of a lamp filamentconstant, comprising: a source of potential connected to the filamentfor providing operating parameters of a given ratio thereto; a pluralityof sensing means connected in series and in parallel with the filament;each sensing means detecting an operating parameter of the filament, andeach deriving respective signal voltages therefrom representative of thesensed parameter; means connecting the respective signal voltages todetecting means for sensing and for deriving an error signal in responseto changes in the ratio of sensed parameters; and means connecting theerror signal to the source of potential to vary the operating parametersand maintain the ratio constant.
 2. The apparatus of claim 1, wherein:the potential connected to the lamp filament is direct current; and theoperating parameters are filament voltage and filament current.
 3. Theapparatus of claim 2, wherein; the means sensing the voltage operatingparameter and the means sensing the current operating parameter are apair of magnetic amplifier control windings; and output windings coupledto the control windings for generating a signal voltage representativeof changes in the ratio of the sensed voltage and sensed current of thelamp filament.
 4. The apparatus of claim 3, further comprising: aconstant current power supply providing the voltage and currentparameters; and a series passing stage in series with the power supplyoutput for detecting the changes in the ratio of sensed voltage tosensed current.
 5. The apparatus of claim 1, wherein: the potentialconnected to the lamp filament is alternating current; and the operatingparameters are filament voltage and filament current.
 6. The apparatusof claim 5, wherein: the means sensing the voltage operating parameterand the means sensing the current operating parameter are a pair ofelectromagnetically coupled transformer windings; and an output windingcoupled to the pair of windings for generating a signal voltagerepresentative of the ratio of the sensed voltage to the sensed currentof the lamp filament.
 7. The apparatus of claim 6, further comprising: Avariable output autotransformer, the input of which is connected to thesource of alternating current the output of which is connected to onepair of windings; and drive means connected to the output of theautotransformer for varying the operating parameters to the transformer.8. THe apparatus of claim 7, further comprising: a phase sensitivedetector connected to the output winding: a source of reference phasevoltage connected to the phase detector; the phase detector combiningthe signal voltage and the reference phase voltage to derive an errorsignal responsive to the change in the ratio of sensed parameters; andmeans connecting the output of the phase sensitive detector to the drivemeans to vary the operating parameters of the lamp filament.